refactor: add proper top-level `__all__` specifiers

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (17282d8) to head (34e736f).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2265 +/- ## ========================================== Coverage 100.00% 100.00% ========================================== Files 63 63 Lines 6889 7122 +233 Branches 1260 1260 ========================================== + Hits 6889 7122 +233 ```

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[CaselIT]

Did you use a script of some sort?

[vytas7]

I just used Python REPL :slightly_smiling_face: But we should use AI I suppose.

Like this:

>>> import falcon.status_codes
>>> for name in dir(falcon.status_codes):
...     if not name.startswith('_'):
...         print(f'from falcon.status_codes import {name}')
... 
from falcon.status_codes import HTTP_100
from falcon.status_codes import HTTP_101
<...>